Optical beam induced current (OBIC) imaging is widely employed for failure or defect detection of pn junctions, inter-level shorts, transistor states, etc, in integrated circuits (IC). An OBIC image is a map of the current magnitudes that are induced when a (focused) optical beam is scanned across an IC sample. Scanning confocal microscopy with its focused probe beam, is readily combined with OBIC imaging to produce a pair of confocal reflectance- and OBIC images of the sample from one and the same beam scan.
The one-photon absorption OBIC (1P-OBIC) is produced by an illuminated semiconductor material if the probe photon energy exceeds the semiconductor bandgap Eb i.e. λp≦hc/Eb, where λp is single-photon wavelength, h is the Planck's constant and c is the speed of light in vacuum. 1P-OBIC is proportional to the probe beam intensity and the measured 1P-OBIC signal is an integrated effect along the optical beam path. Unlike confocal images which are high-contrast displays of the reflectance of a three-dimensional sample, the corresponding 1P-OBIC image of the same sample has low contrast and lacks vertical resolution.
Two-photon OBIC (2P-OBIC) has been demonstrated to generate high-contrast images of semiconductor sites in an IC. 2P-OBIC utilizes an excitation beam with a wavelength λ2P>hc/Eb. 2P-OBIC is proportional to the square of the beam intensity and is highly localized within the focal volume of the excitation beam. Another technique for obtaining high-contrast 1P-OBIC images is via near-field microscopy with a subwavelength fiber Up. A major drawback of 2P-OBIC is the high cost of a femtosecond laser source. Image generation in near-field microscopy is slow and unsuitable for generating large image fields. It is also sensitive to ambient experimental conditions.
Here, we present a procedure for generating high-contrast images of semiconductor sites in the IC from their 1P-OBIC image and confocal reflectance image which are both obtained from the same focused beam. The procedure utilizes the following properties: (1) only semiconductor materials produce an OBIC signal, and (2) confocal reflectance images are optically-sectioned images of both metallic and semiconductor surfaces. We show that the product of the low-contrast 1P-OBIC image and the confocal image results in a high-contrast (axial-dependent) map that reveals only the semiconductor sites in the confocal image. Similarly, the product of the complementary to the 1P-OBIC image and the confocal image yields an optically sectioned image exclusively of the non-semiconductor sites in the IC sample.
Another advantage of 2P-OBIC imaging over 1P-OBIC is realized when observing in the presence of an intervening highly scattering medium between the focusing lens and the semiconductor material. Because the scattered intensity is inversely proportional to a power of the incident wavelength and that λ2P=2λp, a much greater percentage of the 2P excitation photons is delivered at the focal volume of a 2P excitation beam than their 1P counterparts for the same scattering medium and numerical aperture (NA) of the focusing lens. The scatter-induced broadening of the axial distribution of the 2P-OBIC signal is less severe than that of 1P-OBIC. In 1P fluorescence excitation microscopy with large-area photodetector, the effect of scattering is to degrade the signal-to-noise ratio of the generated images.
It is worth noting that confocal microscopy is also robust against the unwanted effects of scattering by an intervening medium. The photodetector pinhole acts a spatial filter that permits only the detection of photons emanating from the focal volume of the probe beam. The undesirable image contribution of the photons from the out-of-focus planes can be minimized through careful choice of the pinhole size.
1P excitation (1PE) confocal microscopy can be done with objectives of relatively low NA values but long working distances—an advantage that is of practical importance for wide-field observation and when dealing with thick samples. In contrast, 2PE imaging requires objectives with large NA values to generate sufficiently high intensities at the focal spot because the 2PE absorption cross-section is much smaller than its 1PE counterpart. Such objectives however, normally have short working distances that limit our ability to scan axially thick samples at long depths. Aberration-free high NA objectives with long working distances are quite expensive to manufacture.